How to add multiple PBR textures on one game-ready object
You're probably talking about the trim sheet method where one material can be referenced by multiple parts of the same mesh. With the way you've UVW unwrapped that model, trim sheets wouldn't work too well. You'd need to straighten those circles into straight pieces. A trim sheet works by tiling but those pieces won't tile well at all. I highly suggest downloading a free add-on called TexTools which will straighten things like that out for you so you can then use them better with trim sheets.
I've created a short video demonstrating the best way to go about this, with your particular model.
Hope this helps!
Using PBR textures with tileable textures.